The invention relates to generating commands for a downhole tool.
Referring to FIG. 1, for purposes of measuring characteristics (e.g., formation pressure) of a subterranean formation 31, a tubular string 10 may be inserted into a wellbore which extends into the formation 31. In order to test a particular region, or zone 33, of the formation 31, the string 10 may include a perforating gun 30 that is used to penetrate a well casing 12 and form fractures 29 in the formation 31. To seal off the zone 33 from the surface of the well, the string 10 typically includes a packer 26 that forms a seal between the exterior of the string 10 and the internal surface of the well casing 12. Below the packer 26, a recorder 11 of the string 10 takes measurements of the formation 31.
The tool 21 typically has valves to control the flow of fluid into and out of a central passageway of the string 10. An in-line ball valve 22 is used to control the flow of well fluid from the formation 31 up through the central passageway of the test string 10. Above the packer 26, a circulation valve 20 is used to control fluid communication between an annulus 16 surrounding the string 10 and the central passageway of the string 10.
The ball valve 22 and the circulation valve 20 can be controlled by commands (e.g., "open valve" or "close valve") that are sent downhole. Each command is encoded into a predetermined signature of pressure pulses 34 (FIG. 2) transmitted downhole to the tool 21 via hydrostatic fluid present in the annulus 16. A sensor 25 of the tool 21 receives the pressure pulses 34, and the command is extracted. Electronics and hydraulics of the string 10 then operate the valves 20 and 22 to execute the command.
For purposes of generating the pressure pulses 34, a port 18 in the casing 12 extends to a manually operated pump (not shown). The pump is selectively turned on and off by an operator to encode the command into the pressure pulses 34. A duration T.sub.0 (e.g., 1 min.) of the pulse 34, a pressure P.sub.0 (e.g., 250 p.s.i.) of the pulse 34, and the number of pulses 34 in succession form the signature that uniquely identifies the command.